Manning

Thermal Control

Robert Manning

AAE450

Spring 2007

Outline

 Fundamentals

 Thermal Control Devices

 Heat Shield (TPS)

 Resources & Considerations

Fundamentals:

 Steady-state thermal modeling is

simply an energy balance.

 Q is heat flux or transfer (Watts)

 q is heat flux per unit area (W/m2)

 Area is ALWAYS normal to transfer.

 Three method of heat transfer:

radiation, conduction, & convection.

Fundamentals: Conduction

 Simple one dimensional condition:

dT T1  T2

qK

 

q12 K

dx x

 K = Thermal conductivity (W/m/K)

 dt/dx = Temperature gradient (K/m)

 Derivative can be approximated

using two temperature (T1 and T2)

Fundamentals: Convection

 Newton’s Law of cooling:



q12  h(T1  T2 )



 h = Transfer Coefficient (W/m2-K)

 Empirical equation. Use Nusselt number

correlations to determine h.

 Laminar/Turbulent?

Free convection/external/internal?

Boiling/Condensation?

Fundamentals: Radiation

qout   T

 4

qabsorb  qincident

 

 Heat emitted is governed by Stefan-

Boltzmann Law.  is emissivity.  is

5.67x10-8 J/(K4-m2-s)

 Heat absorbed is governed by the

absorbitivity coefficient .

 Use view factor relationship (Incropera

Chapter 13)

Fundamentals: Tricks

 Area is projected area of radiation.

 If no heat is generated in body,

temperature can be controlled by

examining /.

 We can treat thermal conductance

as an electrical resistor:

x T

R Q

KA R

Thermal Control Devices

 Passive Thermal Control:

System without any moving parts

or electrical input



 Active Thermal Control:

Anything that has moving parts

and/or electrical input

Multi-layer Insulation

Outer Cover

Reflector

Spacer ………………………………

……………………………… Cover &

Spacer

Structure

 MLI is typically part of micrometeorite

protection.

 Use Effective Emmittance(~0.005):

 *A(T  T )  Q

4

H C

4





 Chapter 13.2.5 from Incropera

Pumped-Loop Systems

 Active Control

 Transfers heat from one location to

another using a pumped liquid.

 Typically use water for human habitat.

 Ammonia or Freon used for external or

non-habitat portions.

 Use counter-flow heat exchangers!

 Chapter 11 of Incropera

Radiators

 Active Control

 Used in conjunction with pumped-

loops to radiate heat into space.

 Two types:

body-mounted or deployable

 Use Flash Evaporators when not

deployed

Thermal Protection System

 Difficult. Ask Prof. Schneider!

 Establish characteristics of entry:

Velocity-altitude profile

bluff or streamlined body

Knudsen number

ablative vs. no ablation

 Consider using existing data or

codes!

TPS: Flow characteristics

 Chemical reaction at high temperatures

 Oxygen: T > 2000 K, Nitrogen: T > 4000 K

 Possible ionization

 Turbulent, separated, shock interactions

 Convection vs. Radiation

 Knudsen: kn > 0.1 => no continuum

   mean free path

kn 

   characteric length

Resources: Books

1) Excellent Thermal Design Book:

David G. Gilmore. Spacecraft Thermal

Control Handbook.

2) Incropera, DeWitt, et al. Fundamentals

of Heat and Mass Transfer.

3) Anderson, John. Modern Compressible

Flow or Hypersonic and High

Temperature Gas Dynamics.

Resources: Web

 Code for aero-thermal modeling:

http://roger.ecn.purdue.edu/~aae450s/

methods.pdf

 TPSX:

http://tpsx.arc.nasa.gov/

Resources @ Purdue

 SODDIT:

Sandia One-Dimensional Direct and

Inverse Thermal Code

 Newton’s Method:

Predicts Cd and Cl for high mach numbers

 Prof. Schneider